Fix Google Books search cache failures (#958)

Recognises google error 503s as invalid results and does not add to
results cache.

Fixes #945

shelfmark/metadata_providers/googlebooks.py

@@ -47,6 +47,11 @@ _HTTP_STATUS_NOT_FOUND = HTTPStatus.NOT_FOUND
 
 GOOGLE_BOOKS_BASE_URL = "https://www.googleapis.com/books/v1"
 
+
+class _GoogleBooksRequestError(Exception):
+    """Raised when Google Books does not return a usable API response."""
+
+
 # Sort mapping - Google only supports "relevance" and "newest"
 SORT_MAPPING: dict[SortOrder, str | None] = {
     SortOrder.RELEVANCE: None,  # Default, no param needed
@@ -117,7 +122,10 @@ class GoogleBooksProvider(MetadataProvider):
             f"{options.query}:{options.search_type.value}:{options.sort.value}:"
             f"{options.language}:{options.limit}:{options.page}:{fields_key}"
         )
-        return self._search_cached(cache_key, options)
+        try:
+            return self._search_cached(cache_key, options)
+        except _GoogleBooksRequestError:
+            return []
 
     @cacheable(
         ttl_key="METADATA_CACHE_SEARCH_TTL",
@@ -166,18 +174,19 @@ class GoogleBooksProvider(MetadataProvider):
         if options.language:
             params["langRestrict"] = options.language
 
+        result = self._make_request("/volumes", params)
+        if result is None:
+            raise _GoogleBooksRequestError
+
         books: list[BookMetadata] = []
         try:
-            result = self._make_request("/volumes", params)
-            if result:
-                items = result.get("items", [])
+            items = result.get("items", [])
+            for item in items:
+                book = self._parse_volume(item)
+                if book:
+                    books.append(book)
 
-                for item in items:
-                    book = self._parse_volume(item)
-                    if book:
-                        books.append(book)
-
-                logger.info("Google Books search '%s' returned %s results", query, len(books))
+            logger.info("Google Books search '%s' returned %s results", query, len(books))
 
         except Exception:
             logger.exception("Google Books search error")

tests/metadata/test_googlebooks_parse.py

@@ -1,6 +1,59 @@
+import requests
+
+from shelfmark.core.cache import get_metadata_cache
+from shelfmark.metadata_providers import MetadataSearchOptions
 from shelfmark.metadata_providers.googlebooks import GoogleBooksProvider
 
 
+class _GoogleBooksResponse:
+    def __init__(self, payload):
+        self._payload = payload
+
+    def raise_for_status(self):
+        return None
+
+    def json(self):
+        return self._payload
+
+
+class _FlakyGoogleBooksSession:
+    def __init__(self):
+        self.calls = 0
+
+    def get(self, *args, **kwargs):
+        self.calls += 1
+        if self.calls == 1:
+            raise requests.Timeout
+        return _GoogleBooksResponse(
+            {
+                "items": [
+                    {
+                        "id": "volume-1",
+                        "volumeInfo": {
+                            "title": "Recovered Book",
+                            "authors": ["Alice Author"],
+                        },
+                    }
+                ]
+            }
+        )
+
+
+def test_googlebooks_search_does_not_cache_request_failures():
+    get_metadata_cache().clear()
+    provider = GoogleBooksProvider(api_key="test-key")
+    session = _FlakyGoogleBooksSession()
+    provider.session = session
+    options = MetadataSearchOptions(query="Recovered Book")
+
+    assert provider.search(options) == []
+
+    result = provider.search(options)
+
+    assert session.calls == 2
+    assert [book.title for book in result] == ["Recovered Book"]
+
+
 class TestGoogleBooksParseVolume:
     def test_parse_volume_returns_metadata_for_valid_payload(self):
         provider = GoogleBooksProvider(api_key="test-key")